1. Field of the Invention
This inventon relates to a timing system for a track following servo system of the type employed with magnetic data storage disk packs having servo data recorded in the form of discrete prerecorded magnetic transitions and more particularly to a timing circuit that achieves accurate clock pulse generation notwithstanding defects in the prerecorded servo tracks.
2. Description of the Prior Art
U.S. Pat. Nos. 3,534,344 and 3,691,543 disclose track following servo systems in which error signals are generated when a servo head is displaced from a path intermediate two oppositely poled servo tracks to restore the servo head to the path. Although the patented systems have met with substantial success, their sensitivity to imperfections in the prerecorded servo tracks imposes unduly stringent requirements on the quality of the disk pack servo surface and the servo data recorded thereon.
Also included in the prior art is a circuit for filling in for dibits missing from the servo tracks to avoid loss of clock synchronization in the absence of one or more dibits. Such prior art circuit includes a counter that is clocked by the system clock pulse and is arranged to produce a preselected count between successive dibits. The counter is reset by a dibit or by a counter output produced at the end of the preselected count so that the counter continues to cycle even in the absence of dibits supplied to the reset input thereof. Such prior art circuit employs only one counter which is reset by dibits from adjacent odd and even servo tracks. Accordingly, the prior art circuit is sensitive to shifts in position of the dibits in one track relative to the position of dibits in the adjacent track, in consequence of which the sync pulses produced by the counter output occur at irregular intervals and thus produce inaccurate clock synchronization.